This invention relates to a process of imparting a predetermined curvature to a sheet of thermoplastics material in which a said thermoplastics sheet is bent by placing it against a face of a bending form having a moulding surface area of the required curvature, heating said thermoplastics sheet and causing or allowing it to conform to said moulding surface area. The invention also relates to apparatus for imparting a predetermined curvature to a sheet of thermoplastics material.
Glass-plastics laminated panels are often used when the panel is apt to be subjected to impact. Thermoplastics sheets in the interior of such a panel can serve to absorb energy on impact against the panel, so giving greater protection to an enclosure incorporating such a panel as a window. If a thermoplastics sheet forms an external ply of the panel, it will serve to inhibit glass fragments flying from that face of the panel on breakage, so reducing the risk of injury from flying glass. Curved panels of that kind may be used as windscreens or other windows of vehicles of various types, including aircraft and motor cars.
It is of course well known to use a thin flexible film of thermoplastics adhesive material, such as polyvinylbutyral, for forming a laminate, but there is an increasing tendency to make use of rather more rigid thermoplastics materials since these have improved properties for impact resistance. Such panels are sometimes referred to as being "bandit glazings" or "bullet-proof glazings", and they can afford a degree of protection against the discharge of firearms.
Polycarbonates and methacrylates are examples of rather rigid thermoplastics materials. Because these materials are very much stiffer than materials which are conventionally laminated to glass, such as polyvinylbutyral, a considerable problem arises in that if the thermoplastics material does not conform to the curved glass sheet to which it is to be bonded, restoring forces in the thermoplastics material of a laminate tend to tear it away from the glass and delamination results. The problem is found to be particularly acute where the laminate has a small radius of curvature, and also around the edges of the laminate, where stresses are concentrated. It is doubly unfortunate in this respect that vehicle windscreens often seem to have their sharpest curvature at or near their edges.
It is accordingly desirable to bend the thermoplastics sheet material into the desired predetermined curvature before lamination. The thermoplastics material may be heated and allowed to sag into conformity with a bending form of appropriate size and shape. But this does not solve the problem of delamination, especially around the margin of the panel, because we have found that as the sheet of thermoplastics material cools after bending, it tends to curl up around its margin, so that it will not there conform to the required curvature. Such lack of conformity also increases the difficulties in forming a good bond between the sheets in a laminating process. Such curling may be pressed out naturally during the laminating process, but residual stresses remain in the thermoplastics material tending to cause delamination around the laminate margins. Even if the thermoplastics sheet is to be used on its own, without lamination, such edge curling is undesirable because it results in optical distortion at the sheet margins, and it makes framing more difficult.
It is a principal object of this invention to provide a process of imparting a predetermined curvature to a sheet of thermoplastics material which alleviates, and in preferred embodiments, eliminates this problem.